1. Field of the Invention
The present invention relates to β-form tris-(2,3-epoxypropyl)-isocyanurate crystals and a process for their production. Particularly, it relates to a process for producing such crystals efficiently as a product of high purity wherein the content of α-form tris-(2,3-epoxypropyl)-isocyanurate which is present on the surface of the crystals in a form to be extracted by an alcohol, is not more than 2 wt %, and epichlorohydrin and an organic solvent hazardous to human bodies or to applications to electronic materials, are reduced to a level of not more than 1,000 ppm.
2. Discussion of Background
In view of an increasing demand in recent years for the properties required for a solder resist material, such as adhesion, electrical insulating properties, soldering heat resistance and solvent resistance, a solder resist ink composition is presently used which is a combination of a photosensitive prepolymer and a thermosetting resin. Namely, it is designed to satisfy the above required properties by forming a solder resist pattern by the photosensitive prepolymer, followed by thermosetting. Further, demands have been increasing for high densification of printed circuit boards along with a trend for light weight and miniaturization of electronic appliances in recent years, for low bleeding during formation of solder resist patterns for surface mounting of parts and for precision in embedding between circuits. Accordingly, as the thermosetting resin to be incorporated to the solder resist ink, a fine particulate solid epoxy having high solvent resistance is desired.
As a solid epoxy to satisfy the above required properties, tris-(2,3-epoxypropyl)-isocyanurate may be mentioned. Tris-(2,3-epoxypropyl)-isocyanurate has three asymmetric carbon atoms, and crystals made of an equimolar mixture of (2R,2′R,2″R)-tris-(2,3-epoxypropyl)-isocyanurate and (2S,2′S,2″S)-tris-(2,3-epoxypropyl)-isocyanurate, wherein all of the three asymmetric carbon atoms are optically isotropic, are commonly called β-form crystals and known to give crystals having a high melting point of a level of about 150° C. This is attributable to the fact that a pair of these two types of enantiomers form a molecular lattice having firm six hydrogen bonds and thus form a crystal lattice. On the other hand, crystals made of a mixture of (2R,2R,2S)-tris-(2,3-epoxypropyl)-isocyanurate and (2S,2S,2R)-tris-(2,3-epoxypropyl)-isocyanurate, wherein one of the three asymmetric carbon atoms is different in the optical anisotropy, are commonly called α-form crystals, and they do not have the above crystal structure and accordingly present only a low melting point of a level of about 100° C. The β-form tris-(2,3-epoxypropyl)-isocyanurate crystals not only have a high melting point but also have a low solubility in various solvents. Accordingly, when they are used as a crosslinking agent for different types of compounds or for reactive polymers in the form of a one pack type reactive mixture, the reaction will not proceed during the storage, until they are forcibly cured. Such β-form crystals have been used for applications to electric and electronic materials, for example, as a solder resist ink composition of photocuring/thermosetting combined type.
The liquid epoxy composition is likely to undergo an increase in the viscosity during storage, since a part of the epoxy compound dissolves in the solvent, and entanglement with the photosensitive prepolymer is likely to result, whereby elution tends to be poor during washing off of the non-exposed portion. JP-B-7-17737 discloses use of β-form tris-(2,3-epoxypropyl)-isocyanurate as a hardly soluble epoxy compound. β-form tris-(2,3-epoxypropyl)-isocyanurate fine particles which have a high melting point and which are hardly soluble, are in a state enclosed by a photosensitive prepolymer, whereby they will not reduce the solubility of the photosensitive prepolymer at the non-exposed portion. Further, they are hardly soluble in an organic solvent, whereby the exposed portion is hardly eroded by a developer, whereby there will be no deterioration in the sensitivity. Further, the storage stability of the solder resist ink composition is excellent.
As a method for separating β-form tris-(2,3-epoxypropyl)-isocyanurate and α-form tris-(2,3-epoxypropyl)-isocyanurate from tris-(2,3-epoxypropyl)-isocyanurate, a separation method has been available wherein a solvent which dissolves α-form tris-(2,3-epoxypropyl)-isocyanurate relatively well and which hardly dissolves β-form tris-(2,3-epoxypropyl)-isocyanurate, for example, an alcohol such as methanol, is employed. For example, Journal of Thermal Analysis, vol.36 (1990) p.1819 discloses separation by means of a methanol solvent. Further, Plaste und Kautschuk 23 Jahrgang Heft 4/1975 discloses a method wherein firstly a methanol solvent is used for separating β-form tris-(2,3-epoxypropyl)-isocyanurate, and then the β-form tris-(2,3-epoxypropyl)-isocyanurate is purified by chloroform. Further, Kobunshi Ronbunshu (polymer report collection), vol.47, No.3 (1990) p.169, discloses a method wherein synthesized tris-(2,3-epoxypropyl)-isocyanurate is put into methanol, followed by heating and stirring, whereupon the non-dissolved content is collected by filtration, and the obtained non-dissolved substance is re-crystallized from methyl ethyl ketone to obtain β-form tris-(2,3-epoxypropyl)-isocyanurate crystals.
Many of β-form tris-(2,3-epoxypropyl)-isocyanurates obtained by such separation methods, hardly undergo crystal growth, and many of them have a small particle size, whereby the filtration operation in the filtration step tends to be very difficult. Accordingly, it is undesirable that the crystals obtained by recrystallization are too fine.
Further, by a single separation operation by the foregoing separation method, β-form tris-(2,3-epoxypropyl)-isocyanurate crystals tend to contain the solvent for recrystallization, chlorine-containing impurities or other impurities. Accordingly, it will be necessary to remove them by further carrying out recrystallization or by melting the crystals once. Particularly, if the remaining organic solvent is not adequately removed, when the tris-(2,3-epoxypropyl)-isocyanurate is used for e.g. a solder resist material, holes formed by evaporation of the solvent are likely to form on the surface of a printed circuit board, and original properties of the resist material can not be adequately obtained. Further, there may be a problem in an application in which surface smoothness is required. Further, in a case where the remaining organic solvent is a halogenated hydrocarbon, it is not suitable for applications to electronic materials. Further, in a case where the remaining organic solvent is a protic organic solvent, storage stability of a composition may be impaired by proton in some cases.
JP-B-48-24039 discloses a process wherein a chlorohydrin ester of isocyanuric acid obtained by reacting cyanuric acid with epichlorohydrin, is dehydrochlorinated with an alkali, the alkali metal chloride thereby formed is separated, and the obtained epichlorohydrin solution of tris-(2,3-epoxypropyl)-isocyanurate is concentrated to a tris-(2,3-epoxypropyl)-isocyanurate concentration of from 50 to 60%, and then the solution is cooled to from 20 to 25° C. to obtain tris-(2,3-epoxypropyl)-isocyanurate crystals in an yield of 27% based on cyanuric acid.
The tris-(2,3-epoxypropyl)-isocyanurate obtainable by a conventional process is known to contain α-form tris-(2,3-epoxypropyl)-isocyanurate and β-form tris-(2,3-epoxypropyl)-isocyanurate in a ratio of 3:1.
The yield of β-form tris-(2,3-epoxypropyl)-isocyanurate present at the reaction stage of JP-B-48-24039 is expected to be at most 20% based on cyanuric acid, and at the stage after the crystallization, the yield is expected to be at most 19% based on cyanuric acid. Whereas, the tris-(2,3-epoxypropyl)-isocyanurate crystals obtained in JP-B-48-24039 is 27% in yield based on cyanuric acid, from which the proportion of α-form tris-(2,3-epoxypropyl)-isocyanurate in the obtained crystals is calculated to be at least (27%-19%)/27%×100=30%. The results of a duplication test carried out by the present inventors also showed that the content of α-form tris-(2,3-epoxypropyl)-isocyanurate was at least 30%. It is considered that in the crystals obtained in JP-B-48-24039, a substantial amount of α-form tris-(2,3-epoxypropyl)-isocyanurate is attached on the surface of β-form tris-(2,3-epoxypropyl)-isocyanurate crystals or is present in the form of independent crystals.
Thus, the above process has a problem that the crystals contain a large amount of α-form tris-(2,3-epoxypropyl)-isocyanurate in the form to be extracted by an alcohol, and further a few thousands ppm of epichlorohydrin, etc. are contained in the interior of the crystals. Namely, as mentioned above, alcohol-soluble α-form tris-(2,3-epoxypropyl)-isocyanurate is present on the surface of the β-form tris-(2,3-epoxypropyl)-isocyanurate crystals or is independently present, whereby there is a problem that it dissolves in a solder resist composition to deteriorate the storage stability or to deteriorate the developability. Further, epichlorohydrin is composed of a hydrolyzable chlorine which is not only hazardous to human bodies but also hazardous to applications to electronic materials, and should be contained as little as possible.
Further, in a process to obtain tris-(2,3-epoxypropyl)-isocyanurate crystals by crystallization from a reaction solution using epichlorohydrin as a reaction solvent, there are following problems is safety in view of operation since a large amount of epichlorohydrin is handled as a solvent. Namely, an operator may be exposed to vapor of epichlorohydrin when seed crystals are charged in a dissolved state, and an operator may be exposed to vapor of epichlorohydrin during operation to repair clogging of a filter generated in a step of filtrating the crystals.
Further, hydrolyzable chlorine is required to be as little as possible in a field in which a high quality is required, however, epichlorohydrin tends to be contained in the interior of the crystals since crystallization is carried out from epichlorohydrin, and it is not easy to reduce epichlorohydrin to be not more than 100 ppm for example.